a) Field of the Invention
This invention relates to a real image mode finder optical system.
b) Description of the Prior Art
The real image mode finder optical system can bring about a large magnification ratio from an adequate design, but nevertheless can diminish an effective aperture of the lens system and particularly the diameter of a front lens, so that it is largely used for a zoom finder. Further, the real image mode finder optical system needs some optical element to erect an image, which is broadly classified as a prism or a relay lens.
The variable magnification finder set forth in Japanese Patent Preliminary Publication No. Hei 1-131510, for instance, is the zoom finder making use of a Porro prism to erect the image. This arrangement is made so that the change of magnification is accomplished by a second lens unit and the correction of dipter by a thrid lens unit, thus having two moving lens units.
In addition to this, the use of a Dach prism and a mirror according to the optical path length and the layout of the arrangement is also known.
Also, like the variable magnification finder stated in Japanese Patent Preliminary Publication No. Hei 1-197717, the arrangement in which the correction of diopter is performed by a first lens unit and the change of magnification by a second lens unit is also available. Further, in the case of the mounting of the optical system, the method using the relay lens brings about increase of the overall length and therefore causes often the optical path to be bent by the mirror, with a view to incorporating effectively the optical system in a camera, like the finder stated in Japanese Utility Model Preliminary Publication No. Sho 64-13034, although not used exclusively for the finder. In this example, the lens system provided in front of the mirror is also used as a photographic lens system so that the change of magnification and the correction of diopter are attained by moving the plural lens units of the photographing lens system.
Each arrangement, however, described in the Hei 1-131510, Sho 64-13034 and Hei 1-197717 has two or more moving lens units as mentioned above and requires a driving mechanism such as a cam with resultant complicated design and manufacture.
In particular, the optical system employing the relay lens as described in each of the Sho 64-13034 and Hei 1-197717 requires to turn back the optical path when being intended for compaction, with the result that a return mirror will be required and the number of parts will be increased.
Although the number of parts may be reduced if the return mirror is moved to become a compensator, the direction of movement of the return mirror is complicated because the optical path is intricate and a device of avoiding the interference of the parts to be moved is required, thus making the design and manufacture difficult.
As such, it is proposed that a variable focal length lens, although not used yet in the real image mode finder optical system, is adopted in place of the moving lens unit.
For example, the variable magnification finder optical system set forth in Japanese Patent Preliminary Publication No. Sho 62-56918 is such that the lens units for the correction of diopter are replaced by the variable focal length lens into a single moving lens unit. Moreover, in each of an inverse Galilean finder stated in Japanese Patent Preliminary Publication No. Sho 61-77820, the variable magnification finder in Japanese Patent Preliminary Publication No. Sho 61-221720, and the lens in Japanese Patent Publication No. Sho 61-50281, the lens units for the change of magnification and the correction of diopter are replaced by the variable focal length lens to dispense with the moving lens units.
The foregoing Sho 61-77820, Sho 61-221720, and Sho 61-50281, however, involve problems that, since the change of the refracting power is made by the variation of the lens configuration, high electric energy will be consumed in order to maintain the lens configuration while being changed and deformation by gravity will be brought about. Furthermore, the arrangement stated in each of the Sho 61-77820 and Sho 61-221720 has encountered difficulties that, unless the lens has the transmittance equivalent to optical glass since it is necessary to be constructed with material corresponding to the thickness of the lens, the visual field of the finder colors, becomes dark, and fades.
Hence, the variable focal length lens designed so that the refracting power is changed by making use of the birefringent property of a liquid crystal to adjust a voltage applied from the external has been already proposed by Japanese Patent Preliminary Publication Nos. Sho 52-32348, Sho 54-99654, and Sho 59-224820.
As an example, a variable focal length liquid crystal lens stated in the foregoing Sho 59-224820 is used in such a way that the vibrating direction of a polarizing plate coincides with the direction of the longitudinal axis (optic axis) of liquid crystal molecules in an initial orientation of the liquid crystal. It follows from this that, in the state where the voltage is not applied, the liquid crystal lens exhibits the refracting power with respect to an extraordinary ray, while in the state where the voltage is sufficiently applied, enclosed liquid crystal molecules rotate to become parallel with the electric line of force and an apparent refracting power changes so that the lens exhibits the refracting power with respect to an ordinary ray. Further, in the intermediate application of voltage, it is possible to change continuously the refracting power in accordance with the value of the applied voltage.
Also, in Japanese Patent Preliminary Publication No. Sho 62-170933, a variable focal length mirror lens is proposed as not a transmission type but a reflection type.
Such a variable focal length lens utilizes its birefringent property, not to speak of the liquid crystal lens, and therefore is combined with the polarizing plate in general. Whereby, ordinary and extraordinary rays are separately used to derive the effect of the variable focal length. Unless the polarizing plate is employed, in the case of a positive lens by way of example, the light subjected to the refracting power relating to the ordinary ray and the light subjected to the refracting power relating to the extraordinary ray form two focal points, resulting in generation of a double image. For this reason, the polarizing plate has been indispensable for most of the variable focal length lenses.
In such conventional variable focal length lenses, however, there has been the defect that light (the amount of light) usable for the polarizing plate is reduced to a half of the required amount, with the resultant narrow range of use. For instance, the finder optical system set forth in the above Sho 62-56918 has had the defect that the visual field of the finder becomes dark because the polarizing plate is employed for the variable focal length lens.
Further, when light deviating from the direction of polarization is made incident, this optical system may change to the optical system having the same characteristics as the one including a polarizer and an analyzer. In such a case, the strain of the lens and the like of the optical system may also be viewed, so that there is the demand of general use of non-polarized light. The lens involving the conventional polarizing plate has been unable to fill this demand.
Moreover, a lens made with a birefringent material so that the optical axis of the lens is not parallel with the optic axis of the material has such a defect as to exhibit astigmatism even on the axis and a lens configuration free of aberration has been demanded.
In addition, any of the optical systems stated in the Sho 62-56918, Sho 61-77820, and Sho 61-221720 is based on the Galilean optical system or the inverse Galilean optical system and has also encountered the problem that, since the effective aperture becomes large with increasing magnification ratio, the optical system cannot be used for the design of a high magnification ratio.